The chain reaction which causes massive damage to the brain following a stroke could be halted by an experimental drug.

So far, the drug has only been tested on animals, but results are promising, and scientists are hopeful that it might be effective in humans as well.

A stroke is caused when a blood vessel supplying part of the brain suffers a blockage or ruptures.

The brain cells are starved of the vital oxygen that the blood carries, and start to die.

However, other cells in the same area often die, even though there is no apparent reason for this to happen.

Effectively, many more cells "commit suicide" than are actually killed by the direct impact of the stroke itself.

This means that patients suffer far more brain damage than if this process could be halted.

Hour before

Now researchers from two Toronto hospitals believe they may have found a way of stopping this damage in its tracks.

When the drug was given to rats within an hour of a stroke, the damage was halted immediately.

The drug, if given an hour before the stroke, also prevented much of the subsequent damage.

We have not encountered any adverse long-term effects of the drug

Dr Michael Tymianski

It works by affecting receptors on brain cells which have two roles.

If stimulated, the receptors do reduce the amount of brain damage by stopping the cycle of destruction.

However, the same stimulation can also have a dangerous effect on vital brain cell messages.

The Toronto team found a way of stimulating the first action while blocking the dangerous side-effect.

'Over-stimulation'

Dr Michael Salter, one of the researchers leading the project, said: "The drug works by preventing the negative consequences of the over-stimulation of the receptors in the brain that are involved in strokes.

"However, it doesn't block the normal important functions of these receptors, making this a possible practical stroke therapy."

Another investigator, Dr Michael Tymianski, said: "To date, we have not encountered any adverse long-term effects of the drug, and all our data show that the drug is more effective in preventing stroke than any method that has ever been used in animals or humans."

However, he cautioned that it would be some time before human trials could be attempted.

A spokesman for the Stroke Association said that while the study was "interesting", there could be practical problems in getting any eventual treatment to patients in time.

She said: "Unfortunately, the care of stroke patients in the UK is not geared up to treat stroke as a medical emergency, unlike a heart attack, although this is something we would like to change.

"At present, guidelines state that patients should be seen and diagnosed within 48 hours."